A conventional color television tube has a cathode ray tube, three electron guns (i.e., one gun for the red image, a second gun for the green image, and a third gun for the blue image) and a shadow mask or aperture grill which serves to block the three electron beams produced by the guns from hitting the wrong phosphors on an inner surface of a faceplate of the cathode ray tube. While the shadow mask acts as an effective block, it causes some difficulties.
For example, approximately eighty percent of the total electron beam current produced by a gun hits the shadow mask and is dissipated therein as heat. This heating causes the shadow mask to expand. The process is called doming and results in an upper limit on the tube's brightness because as higher electron beam currents are used to achieve greater brightness, more expansion occurs and causes the shadowmask to eventually lose its registration with the phosphors on the faceplate.
The shadow mask also limits the resolution of the display, which depends on the number and size of the holes in the mask. There are plainly only so many holes that one can put in the mask and still keep it stiff. Also, as the hole size decreases, less of the electron beam reaches the phosphor, thus lowering the brightness.
Without a shadow mask, the problems with doming and resolution are eliminated. However, without a shadow mask proper positioning of the electron beam becomes more crucial. To properly position the electron beam, it is necessary to be able to determine and adjust the position of the electron beam.
One technique for controlling the position of the electron beam involves detecting light on the outer surface of the faceplate of cathode ray tube when the electron beam strikes a phosphor. The detected light is then converted to a position signal indicating the position of the electron beam on the faceplate. The position signal is then compared against a desired location signal for the electron beam, an error signal is generated and the error signal is used to correct the positioning of the electron beam. One of the main problems with this technique is that it requires an expansive detection system outside of and separate from the cathode ray tube to control the position of the electron beam which renders it not commercially feasible.
Another technique for controlling the position of the electron beam involves generating an electronic current as the electron beam hits an electrode on the faceplate of the cathode ray tube and then coupling this current out from the cathode ray tube using transformers. One example of such a system is disclosed in U.S. Pat. No. 4,635,107 to Turner, which is herein incorporated by reference.
Although the technique using transformers works, it has problems. For example, the transformers used in this technique are expensive because they must be able to faithfully transform a few microamps of current into detectable levels of currents while successfully withstanding potential differences of twenty-five kilovolts or more. Additionally, the leads from the transformers must pass through the cathode ray tube to get to the current signal from the electrodes out.